The detection of a narrow band signal component, such as a sinusoidal signal in a noise signal, is a problem that often needs to be solved. Different known methods are available to solve this problem. A first method is using correlation calculations, a second is a method based on parametrizing followed by peak picking, and a third is using a number of zero crossing counters.
All these known methods bear the drawback that high computer power is necessary because of the complex algorithms which must be applied. In particular, this is the case if speech signals are being processed. Possible fields of application are telecommunication products, audio products or hearing devices. Under the term xe2x80x9chearing devicexe2x80x9d so-called hearing aids, which are used to correct an impaired hearing of a person, as well as all other acoustical communication systems, for example radio sets, must be understood.
The present invention refers to a method for checking an occurrence of a signal component in an input signal, a use of the method, as well as a hearing device.
To check the occurrence of a signal component in an input signal a method is disclosed, where a measure for the frequency of an input signal is determined, the determined variance is compared with a given limit value, and, the occurrence of a signal component is confirmed if the variance is within a given range in relation to the given limit value.
The method according to the present invention is characterized by a number of very simple method steps, which can be performed by using little computer power. Therefore, the method according to the present invention qualifies in particular for the use in systems having restricted access to energy supply, as for example for mobile devices which must be power line independent, or for systems in which the occurrence of a signal component must be determined very quickly.
In further embodiments of the present invention it is proposed to use the method for the detection and elimination of signal feedback. Signal feedback is a known problem in hearing devices, in mobile telephones and other telecommunication products. A number of solutions have been elaborated by the telecommunication industry. It is known to attenuate the signals in the signal feedback path by corresponding adjustment of the attenuation in the transfer function in the feedback path. Furthermore, the use of auto- and/or cross correlations schemes has been envisioned by which the correlation of the input signal and the output signal are calculated in the time domain or in the frequency domain. The results of the calculations are used to adjust the transfer function in the signal feedback path, using the LMSxe2x80x94(Least Mean Square)xe2x80x94algorithm (feedback canceller). Alternatively, the results of the calculations are used to adjust the transfer function in the forward path, whereby the loop gain is reduced at the critical frequencies.
For further information on the known methods it is referred to the following printings: U.S. Pat. No. 5,680,467, EP-0 656 737, WO 99/26453, WO 99/51059, DE-197 48 079.
The known methods have been used successfully but have the drawback that again a high computer power is necessary to obtain useful results. The use of the known algorithms in hearing devices leads to an increased energy usage. As a result thereof, the operating time until the next recharge or replacement of the batteries is reduced which must be prevented.
In case the loop gain reaches a value which is greater than one in a given frequency range, and in case the magnitude of signal components is some decibels lower at other frequencies than the frequency of the feedback signal if the gain is increased in the forward path, then a notch filter according to the present invention can be used to reduce the signal feedback. In case that different critical frequencies lie too far apart, several notch filters can be used according to a further embodiment of the present invention.
In order that a notch filter can be adjusted to the critical frequency, i.e. the feedback frequency, the critical frequency must be detected first. According to the invention this is performed by the calculation of the variance of the measure for the frequency of the input signal, whereas signal feedback is being detected if the variance lies within a predetermined range in relation to a predetermined limit value.